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Comment by crisdux | Hacker News Reader

Regarding #2, they can choose to decrease flow out of the main spillway and let it go over the emergency spill way - From my understanding, that is exactly what happened over the weekend. They were trying avoiding continued damage to the main spillway.

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Quoting from one of the LA Times updates linked from the link:

The result is a balancing act — drain as much water as possible as quickly as possible while trying to minimize further damage to the spillway. The main spillway was returned to action, but at reduced capacity compared to what it could normally do under the current conditions. After all, the spillway needs to last for the remainder of the rainy season.

The stated capacity of the main spillway is 250,000 cubic feet per second, once it became clear the emergency spillway was not holding up, they increased the main spillway to 100,000 cubic feet per second. So limiting damage to the main spillway is an ongoing concern.

The capacity of the intact spillway is ~250,000 cfs - the capacity of the Feather River through Oroville and Yuba City below, currently in the lower flood stages with 100,000 cfs outflows from the main spillway, is much lower than that. In addition to attempting to minimize further erosion of the main spillway, they set an outflow level that wouldn't flood the towns downstream.

It's difficult to describe the hydrological situation of Yuba City and Marysville to people who have never been there. The rivers run through the towns way up above the street level, owing to mining debris washed down from the mountains in the Gold Rush era, and later reinforced by Corps of Engineers works. So if the rivers overflow their banks in Yuba City, the city will be ruinously flooded.

It's really freaky to stand in downtown Marysville and look up at the river bank 15 feet above you.

On Street View that doesn't appear to be the case at all. It looks like there's a 15-20 foot levee between Marysville/Yuba City and the Feather River.

Do they have release valves where they can release water and potentially cause minor flooding over a wide area instead of potentially catastrophic flooding wherever a "natural" breach occurs?

Both the main and the emergency spillway send water into the Feather river.

When the reservoir is at capacity, the dam operator has no control over how much water goes into the Feather River nor whether downstream towns are flooded or not. Any water not going down the main spillway will go down the emergency spillway and into the river.

(edit for typo)

They can choose to use the emergency spillway. They just can't choose not to use it. That is, if water rises high enough, it will go out the emergency spillway, and they can't necessarily prevent the water from rising that high.

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